1. Field of the Invention
The present invention relates to voltage-dependent resistors. In particular, the present invention relates to a voltage-dependent resistor having high thermal resistance, high surge withstand capability, and stability in its characteristics.
2. Description of the Related Art
As an example of voltage-dependent resistors, Japanese Unexamined Patent Application Publication No. 5-226116 (Patent Document 1) discloses a multilayer varistor using ceramic grain boundary barriers.
This multilayer varistor includes semiconductive ceramic layers that mainly contain ZnO and are doped with bismuth, inner electrode layers, and outer electrodes. The semiconductive ceramic layers and the inner electrode layers are alternately laminated and co-fired. The outer electrodes are disposed on both end surfaces of this composite. One end of each inner electrode layer is alternately electrically connected to either outer electrode.
For another example, Japanese Unexamined Patent Application Publication No. 1-200604 (Patent Document 2) discloses a junction voltage-dependent resistor using a ceramic potential barrier.
This junction voltage-dependent resistor is a ZnO varistor shown in FIG. 4. In FIG. 4, at least one ZnO layer 53 mainly containing ZnO and at least one metal oxide layer 54 are alternately laminated on an electrode 52 disposed on a substrate 51. The ZnO layer 53 and the metal oxide layer 54 are crystallized during their formation to form a potential barrier 55. The ZnO varistor includes another electrode 56 for allowing current to flow through the junction between the ZnO layer 53 and the metal oxide layer 54.
In this ZnO varistor, the potential barrier 55 can be reliably formed because the metal oxide layer 54 is crystallized during its formation. Such a potential barrier 55 improves the nonlinearity and thermal stability of the ZnO varistor. In addition, this ZnO varistor does not require heat treatment after formation of the metal oxide layer 54, preventing cracks on the ZnO layer 53 and the metal oxide layer 54.
The ZnO varistor in Patent Document 1 uses potential barriers that occur at the boundaries between Bi-doped crystal grains to achieve varistor characteristics. This ZnO varistor unfortunately has difficulty in precisely achieving target varistor characteristics because the number of potential barriers and the varistor voltage vary according to, for example, the concentration of oxygen absorbed in the grain boundaries and variations in the diameter of the crystal grains due to the grain growth.
Such a junction voltage-dependent resistor as in Patent Document 2 includes a ZnO layer and a metal oxide layer that are made of thin films deposited by sputtering. Deposition of such thin films needs, for example, a highly clean atmosphere and high vacuum, and involves a difficult defect control of ZnO, which is a semiconductor. In addition, when voltage is applied to the junction voltage-dependent resistor, which is not co-fired, the ZnO layer and the metal oxide layer diffuse into each other at their junction. Furthermore, such thin films readily crack due to the difference in thermal expansion from a substrate on which the thin films are deposited. As a result, unfortunately, the junction voltage-dependent resistor fails to withstand a satisfactory amount of energy, exhibiting an unsatisfactory surge withstand capability.